In power transmissions, a oil reservoir or sump is provided from which a pump draws fluid for delivery to various components within the transmission. The pump sends the fluid to a valve body which distributes the fluid under various pressures to a torque converter and to various transmission components as well as directing oil to a transmission oil cooler.
The transmission oil cooler returns the fluid to the transmission cross section lubrication system wherein various bearings and other rotating devices are lubricated. The transmission sump must contain enough oil to maintain the pump inlet covered and account for transmission oil in transit between the oil cooler and the transmission sump. That is, the oil directed to the lubrication circuit becomes oil in transit once the transmission is put in operation thereby reducing the level of the oil in the sump. Thus, the initial sump fill must be at a sufficient level to account for the in transit oil and maintain the pump inlet covered at all times.
In transit, oil volume is very large at cold temperatures due to the viscosity of the oil. As temperature increases, oil volume in transit decreases as viscosity decreases, thereby increasing oil level in the sump. In prior art devices, this increase in volume is normally accommodated by a secondary storage reservoir or by making the original reservoir sufficiently deep thereby making room for oil expansion.